When a tank or other closed, or essentially closed, space is loaded with a given volume of liquid per time unit, pressure relief valves of the type according to the present invention may be applied to the tank in order to release gas when the pressure within the tank reaches a predefined opening pressure.
Further, pressure relief valves of the type according to the present invention may be configured to release gas when the pressure within the tank reaches a predefined opening pressure as a result of evaporation of or from the liquid in the tank. Additionally, any increase or drop in the ambient temperature during storage or transport may lead to pressure changes within the container. The pressure changes may bring about a certain volume of boiling-off, from product like petroleum products and other chemicals, which may cause the pressure relief valve to open.
The volume vented through the valve, per time unit, defines the capacity of the valve. The capacity depends primarily on the flow resistance through the valve and of the pressure prevailing within the tank.
The capacity of the pressure valve determines the maximally allowable loaded volume per time unit. When, upon interruption of the loading, the pressure within the tank drops to a certain value, i.e. the closing pressure of the pressure valve, the pressure relief valve closes.
The closing pressure of the valve is typically lower than the opening pressure of the valve. In most of the known pressure relief valves, the closing pressure is considerably lower than the opening pressure.
Another use of pressure relief valves applies when, for safety reasons, it is required to supply an inert gas, such as nitrogen, to the free ullage of the container above and/or around the product being transported. For instance, the product may be inflammable liquid or perishable goods such as oranges and lemons that give off inflammable gasses following storage. The inert gas is automatically kept at a predetermined pressure that may be above the atmospheric pressure, but below the opening pressure of the pressure valve. In this case any rise or a drop in the temperature of the surroundings will give rise to a corresponding change in the pressure of the inert gas within the container. This pressure change may be controlled by a pressure relief valve according to one aspect of the present invention.
It has been found that pressure relief valves tend to open and close in a rapid fluctuating or oscillating manner. The fluctuations cause the valve body and the stem to oscillate. This undesirable behaviour cause increase wear and tear on the valve and may cause the velocity through the valve to drop below 30 m/s allowing a flame to travel, or propagate, through the valve.
The fluctuations or oscillations may be caused by the design of the valve or by fluctuations in pressure caused by the/any pipe system between valve and tank or the tank it selves. In other words, the oscillations lead to improper or even dangerous valve operation. This behaviour is hereafter defined as undamped oscillation, or opening or closing, of the pressure relief valve.
WO 0 50 93 302 A discloses a pressure relief valve comprising a housing containing a valve seat and a spring loaded valve body that can be displaced in the housing between open and closed positions. The pressure required to hold the valve body in its open position, above the valve seat, is lower than the pressure required to lift the valve body from the valve seat.
US 2004 177883 A, by the present applicant, discloses a pressure relief valve of the type according to the present invention. The pressure relief valve includes a valve body and a lifting plate. The pressure relief valve is configured such that, at any time, the container to be ventilated is in communication with the area above the lifting plate. The valve further include a magnet and a magnetisable body and the force of attraction Fm between the magnet and the magnetisable body as well as the mass of the lifting plate contribute in defining the abutment force Fc, of the valve body against the valve seat of the valve in its closed state.
U.S. Pat. No. 5,873,384 A also by the present applicant, discloses a pressure relief valve of the type according to the present invention. The valve include a housing and a drop-shaped valve body with a conical valve surface that cooperates with a conical valve seat at an upwardly directed blow-off opening of the valve housing. The drop-shaped body is connected to a lifting disc in the housing. The inner configuration of the housing provides an annular passage around the lifting disc which is smaller in area when the lifting disc is in its uppermost, fully open positioning as compared to its lowermost, closed positioning.
US 2004177883 A as well as U.S. Pat. No. 5,873,384 A are incorporated herein in their entirety.
It is an object of the present invention to provide a pressure relief valve of the kind described above. According to the invention, measures are taken in order to prevent undamped oscillations of the valve body arranged to cooperate with the valve seat arranged in the outflow of the valve.
This is obtained by the pressure relief valve as described in the appended claims.